With the increasing attention now being given to the micro-miniaturization of electronic circuitry, efficient fabrication of miniaturized electrical components, such as resistors and resistor assemblies or arrays, has taken on new importance. One prior art method of making thin film resistor assemblies (U.S. Pat. No. 2,994,846) is initiated by coating the inner surfaces of holes made in a suitable substrate with a thin titanium film. The titanium film is then converted into a high resistivity film by anodizing the film in a bath essentially consisting of an anodizing electrolyte and an etching material capable of etching the metal oxide formed on the titanium film as a result of anodization thereof. The concentration of etching material in the bath is chosen so that the surface of the film is converted into an oxide by anodization before being attacked by the etching material, the time of simultaneous anodizing and etching in the bath determining the resultant resistivity of the film. In a preferred embodiment of the simultaneous anodizing-etching process, a two-bath treatment is provided in which the first bath performs the sinultaneous anodizing and etching of the film as described above until an intermediate resistivity is obtained, then the final value of resistance is obtained in a second bath containing an anodizing material but no etching material. This second bath is chosen so that the anodizing process penetrates to a greater depth than did the anodizing process of the first bath, thereby causing a greater portion of the titanium film to be converted to oxide to increase the resistivity of the film.
The value of the resistors made by the described anodizing-etching process depends upon several factors, namely, (1) the surface area of the holes supporting the titanium, (2) the uniformity of the thickness of the film of titanium deposited on the surfaces of the holes, and (3) the portion of the titanium film converted to an oxide. The second factor, that is, film uniformity, is difficult to control especially when the aspect ratio of the holes, that is, the width to depth ratio of the holes, is large. Film uniformity is especially difficult to control when the film is depositd by an electrolysis deposition, since such a deposition tends to form thicker coatings at the edges of the holes. The third factor, that is, the portion of the film oxidized, is also believed hard to control and sophisticated monitoring apparatus is believed to be required to control what portion of the film is oxidized.